It is well known that the television signal is a very complex signal, requiring precise timing and exacting analysis of its parameters in order to produce high picture quality and maintain it. Accordingly, instruments designed to improve and maintain picture quality for timing, testing, measuring, correcting, and displaying of the video signals are also well known. As an example, a picture monitor used in the television production and transmission facilities is required to present program material and test patterns for critical evaluation by both engineering and production people.
As is also well known, three basic or major television systems are commonly in use: NTSC (National Television System Committee) System; PAL (Phase Alternation Line) System; and the SECAM System which uses sequential chrominance signals and a memory device. Other systems such as ART and NIR exist, but since these systems are primarily not intended to be used for public transmission, they will not be given further consideration.
Basically, the difference between the three primary systems occurs "in the process which is used for transmitting the colouring signals", see "Colour Television" Volume II PAL, SECAM and Other Systems, by P. S. Carnt and G. B. Townsend, copyrighted by P. S. Carnt and G. B. Townsend, 1969, first published in 1969 by Iliffe Books Ltd for WIRELESS WORLD. For the sake of brevity, in the NTSC system, the initiation of the color television signal at the signal source starts with the generation of seven components on a line to line basis: horizontal line synchronization pulses; color sync (burst); set-up; picture detail; color hue; color saturation; and full field synchronization (field synchronization is an exception to the line to line basis). These seven components, when combined to form a continuous waveform, form the composite video signal. Of primary concern in color is, of course, the color burst used to frequency and phase lock the picture color information, color hue and color saturation. Since the television signal is not capable of transmitting color as color, but only as electrical signals proportional to certain characteristics of color, a transformation is made whereby primary red, green and blue video signals are translated into three well known related quantities; luminance, hue and saturation. The transformation is carried out by a resistive matrix to form the luminance (Y Signal) and two color-difference signals (R-Y, B-Y). Transmission of the two color-difference signals is accomplished by amplitude modulating carriers of identical frequency but differing in phase; the separation of the carriers is established at 90 degrees. These two modulated signals are then added together to feed a common transmission channel, forming a sum of the two modulated carriers. In addition, color sync phase (burst phase) of the NTSC System is changed each line with respect to the leading edge of horizontal sync i.e., if fields 1 and 2 have color sync which starts at zero and goes positive on a chosen line, then fields 3 and 4 have color sync which starts at zero and goes negative on the same line. This phase difference is unimportant in the decoding process. However, such phasing has an impact on studio equipment such as time base correctors associated with video tape recorders (VTR's) where it is imperative to be able to detect the phase of color sync or at least know whether or not the phase of burst (or color sync) is proper. That is to say, there are 4 fields in the NTSC signal where fields 1 and 3 (or fields 2 and 4) are identical except that burst is opposite phase on any given line. To verify that two NTSC signals are field synchronized, the line color sequence of burst must be known.
In PAL, the same signals exist as in the NTSC System, but one of the color difference signals (R-Y) is reversed in sign from line to line. Color decoding in PAL is therefore line sequential and the exact sequence has to be known to the decoder to decode the signal properly. This color sequence defines 4 distinct fields in PAL. In addition, burst phase measured with respect to sync also alternates similar to the NTSC signal as is well known. This defines an 8 field sequence. For video processing of two or more PAL signals it is sometimes necessary to synchronize all 8 fields of a plurality of PAL signals.
SECAM also uses the same color difference signals. However, they are encoded into a composite video signal using frequency modulation on a sequential line basis. Since the SECAM lines are transmitted line sequentially i.e., R-Y line followed by a B-Y line etc., it is necessary for the decoder to be able to deduce which line is being transmitted, else the colors of the reproduced picture may not be synchronized with the transmitter. The color sequence and basic synchronization signals are sufficient to define 4 fields of the SECAM signal. The exact phasing of the subcarrier is not yet firmly specified. For present uses, synchronization of these 4 fields of a plurality of SECAM signals appears sufficient for signal processing. Presently, the SECAM signal includes a color synchronizing signal which is transmitted during every field blanking interval to enable the decoder to check the switching sequence. This color synchronizing signal is well known as the Line Identification Signal which consists of 9 lines in the form of a subcarrier across the full width of the line with zero luminance. These line identification signals are transmitted on line 7 to 15 of fields 1 and 3 and on lines 320 to 328 of fields 2 and 4 of the picture. In fields 1 and 2, B-Y is transmitted on odd lines whereas R-Y is transmitted on even lines. In fields 3 and 4, the reverse is true. It should be mentioned that R-Y and B-Y color difference signals are sometimes referred to as D'.sub.R and D'.sub.B respectively. See the already referenced "Colour Television" or "SECAM Colour TV System", Compagnie Francaise De Television, Imprimerie Nord-Graphique.
In SECAM, the use of these line identification signals is a disadvantage however, in that 9 lines in each SECAM field could otherwise be used to provide vertical interval or insertion test signals which could be used for the improvement of the system operation by both engineering and production people as previously mentioned. This problem was considered in my previously filed application, Ser. No. 711,654, filed Aug. 4, 1976 which describes a SECAM decoder which did not require the use of the Line Identification Signals during vertical blanking for decoding purposes. When the vertical interval Line Identification Signals are not used for color sequence identification, the white reference present during the back porch of a SECAM color line must be used. This presents two further disadvantages: (a) within a single television signal the sequence information contained in the field blanking interval may differ from the information contained in the white reference. An operator using the white reference for color sequence could not tell this was the case, so the problem would pass unnoticed, and (b) for multiple sources of SECAM, when the white reference is used for color sequencing, the color sequence lock of all of the sources can not be easily verified before switching. This problem is exacerbated by the proposed deletion of the Line Iidentification Signals present during the vertical interval.
In PAL, most studio sources are gen-locked. Therefore, when a user views two or more PAL signals simultaneously, it is assumed that the PAL signals are synchronized with respect to color sequence. This assumption without verification constitutes a disadvantage. In NTSC, most studio sources have been synchronized with respect to the basic synchronization signals but not with respect to subcarrier phase which is necessary for editing with modern studio processing equipment. This 4 field synchronization of a plurality of NTSC signals is generally verified through the use of a waveform monitor which is a cumbersome procedure. This is, of course, a disadvantage.
Clearly, what is devoid in the prior art, whether NTSC, PAL or SECAM is a means or apparatus and method for testing color sequencing of the color television signals such as testing color sequencing lock between multiple sources or testing for color sequencing information conflicts when multiple sources of determining color sequence exists within a single television signal as outlined above.